In this paper, we have coupled on-line gel electrophoresis to ICP-MS to investigate the separation of iron containing proteins (cytochrome C, haemoglobin, transferrin and ferritin). The GE system has been operated with quite different conditions, using (a) sodium dodecylsulfate (SDS) for charging and denaturing proteins and (b) using native conditions below and above the isoelectric point resulting in anodal and cathodal separations. The loss of the iron metal under each condition is discussed, so that quantification cannot be applied in each mode. For this purpose the ratio of ⁵⁶Fe⁺ to ³²S⁺ was additionally measured.